This invention relates generally to a liquid level measuring arrangement and more particularly concerns a liquid level measuring arrangement using capacitive probes which are placed within reactor tanks and the like.
The chemical industry and the food processing industry use a substantial number of vessels such as reactor tanks. Such reactor tanks are used for the storage and transport of liquids, for the chemical reaction of solids and liquids, and the like.
The structure of the tank is commonly steel so as to withstand the temperature and pressure ranges of the product processing. The products contained within these reactor tanks can be corrosive to bare metal. As protection against corrosion of the tank material and subsequent contamination of the product the interior of the reactor tanks are often enameled with glass or glass ceramics or the like. The resultant glassed steel is resistive to all concentrations of all acids except hydrofluric to 212.degree.F and to most alkalis at moderate temperatures. Many acid solutions having temperatures up to 450.degree.F can be processed. The glass exhibits no catalytic effect in any known case. It does not stimulate undesired side reactions so the product uniformity and yield may be higher than can be achieved with bare metal. Since the glass is inert, the use of glass steel equipment eliminates the possibility of metal contamination. Additionally, the smooth hard surface of glassed steel resists adherence by many products such as synthethic rubber, plastic and resins whch would cling to a bare metal surface.
The common application for the reactor tank is the stirring or mixing of various liquids. To accomplish this purpose an agitator is provided, the agitator is usually driven by an external motor which drives a shaft connected to agitating blades immersed in the liquids. The shaft enters the tank by means of a stuffing box or rotatry seal or the like. A stationary member or baffle, as it is known in the trade, is immersed in the liquid in proximity to the agitating blades to enhance the agitator's action. The agitator stirs the liquid against the baffle which deflects it back to the blades.
It is often necessary to determine the level of the liquid contents of the reactor tank. One approach is to use capacitive probes to gauge the liquid within the tank. The prior art has used one or more elongated electrodes extending from atop the tank into the liquid. If a single probe is used, the capacitance between the probe and the wall of the tank is considered. If two or more probes are used the interprobe capacitance is considered.
In describing the principal of operation, we recall that any pair of electrodes form a capacitor, the value of which is determined by the effective area of the electrodes, the distance between the electrode and the dielectric constant of the media between the electrodes, herein the probe and the tank walls. In gauging the liquid in a tank, the dielectric media will vary depending upon the fluid level. The variance in capacitance is approximately linear with the liquid level of the effective area of the electrodes is constant within the tank. the capacitance of the probe may be measured by a capacitance bridge or the like and so determined. By calculations or calibration the corresponding liquid level in the reactor tank can be deduced from the value of capacitance.
The probe is usually a separate member extending vertically from the top of the tank into the liquid to be measured. The member is commonly a metal rod covered with an insulator. When non-conductive liquids are being measured, the insulator can be eliminated. The probe is usually introduced external to the tank and extended vertically through the tank to be immersed in the fluid. As aforementioned, these tanks are used for a great variety of processes, many of which involve pressure and temperature extremes. As a result, any entry into the tank requires expensive and complicated pressure seals. Additionally, spaces for tank openings or nozzles are at a premimum in reactor vessels and are, therefore, to be avoided when possible.
It would, therefore, be highly desirable if an independently existing member, such as a baffle could also be used as a capacitive probe. For reasons concerning mixing actions and fluid mechanics, however, baffles normally have non-uniform cross sections and therefore the effective area of the capacitor will vary causing non-linear changes in capacitance as the liquid level varies. In some applications, the tank may not be cylindrical and it would be desirable to offer ready means to shape the profile of the probe in accordance with the tank shape to obtain linear response. An ideal probe should be inexpensive yet rugged so as to withstand the abuse encountered under forseeing conditions.
It is, therefore, and object of this invention to provide a new and improved capacitive gauge for measuring the level of a liquid in a tank.
A further object of this invention is to provide new and improved capactive gauge that utilizes an existing member of a tank as a capacitive probe, even where the existing member has a non-uniform cross section and or configuration.
An additional object of the invention is to provide a new and improved capacitive probe having a effective area to compensate for tank shapes.
Another object of this invention is to provide a new and improved enable capacitive probe being rugged, yet inexpensive.